Perimeter testers serve to measure the sensitivity of the retina of the human eye, wherein the sensitivity of the retina is reduced from the center toward the periphery. Thus, the testing point luminous density may by no means be the same all over the field of vision, but must be adjusted to the sensitivity curve of the tested eye. This is achieved in perimeter testers of the type in question here through a step-like gradation of the testing point luminous density.
Decisive for good results during the measuring of the sensitivity of the retina is a good fixation of the patient's eye on the centerpoint of the perimeter tester. This fixation must be maintained during all tests of the sensitivity, particularly during tests toward the periphery. It is known to have an observer constantly check the fixation through a telescope, which in the long run is tiresome. Furthermore, it is known to automatically check the fixation using an infrared transmitter and receiver system, wherein the corneal reflection of the patient is evaluated as a feedback signal. This infrared transmitter and receiver, however, sits exactly in the center of the perimeter ball in order to avoid reception of an oblique incident beam. The disadvantage of this arrangement is that the center of the retina cannot be examined directly, but only through an eccentric fixation, wherein then an automatic fixation control is no longer possible. Aside from these disadvantages, the system is also very susceptible to trouble, because a movement of the head without a change of the fixation leads to an error signal.
Furthermore, it is known to monitor the fixation by means of a television camera, wherein the pupil is adjusted directly with a curser. The smallest deviations can still be found here. This system is very precise, but is also very complicated and thus expensive.
A further known method for determining the fixation is to provide testing points in the blind spot, or in other words at the point at which the vision nerves enter the cortex. The disadvantage of this method is that the blind spot in individual patients is not always at exactly the same spot, and in addition can be of different sizes. This method can for this reason be used successfully only if, prior to the measurements, the blind spot is determined exactly in terms of position and size. This is, however, complicated, and on the other hand the limit values must then in addition be measured.
A basic purpose of the invention is thus to provide a method for testing the fixation of a patient's eye during perimeter measurements, which on the one hand, requires minimal testing effort and, on the other hand, does not have any influence on testing of the retina center and requires an extremely small apparatus cost.